Images: Zooplankton

This squid, called Histioteuthis sp., is about six inches long, and lives in the midwater depths, nearly 1,000 meters (3,000 feet) down. Its skin is covered with spots, called chromatophores, that let it change color at will. Interestingly, its two well-developed eyes are two different sizes.
(Larry Madin, Woods Hole Oceanographic Institution)

Deep-living, transparent, and heart-shaped, this ctenophore (or comb-jelly) is called Thalassocalyce, which means "sea chalice." Like all ctenophores, it is predatory, catching prey with sticky secretions. (Larry Madin, Woods Hole Oceanographic Institution)

This rose-tinted shrimplike crustacean, Thysanopoda obtusifrons, is one of the animals known as krill. Krill are a mainstay of the ocean food web, eaten by larger animals including fish, squids, and marine mammals. (Nancy Copley, Woods Hole Oceanographic Institution)

Alacia valdiviae, an orange ostracod, brought up in the plankton net from deep water. Ostracods are small swimming crustaceans that live within a hinged shell. (Russ Hopcroft, University of Alaska Fairbanks.)

Radiolarians, single-celled marine organisms that drift in the ocean, live together in large, soft, colonies that can be inches across. An assortment the colonies reveals the variety of shapes they can take. (Larry Madin, Woods Hole Oceanographic Institution)

Pyrosomes are colonial tunicates, animals related to sea squirts, that look like fuzzy paint rollers, with individuals arranged around a hollow tubular center. This one has a spiny single-celled animal called a radiolarian, hitching a ride at one end.

A copepod from deep in the Atlantic, Valdiviella insignis. Copepods are among the most numerous animals in the sea and are food for many larger animals. (Russ Hopcroft, University of Alaska Fairbanks.)

A jellyfish-relative called a siphonophore, of the species Rosacea cymbiformis). This one has relaxed and extended some of its many tentacles, which carry batteries of stinging cells to kill prey ranging from plankton to larval and small fish. (Larry Madin, Woods Hole Oceanographic Institution)